Electrochemical machining using a chloride electrolyte including 2-mercaptobenzothiazole or 2-benzimidazolethiol



United States Patent 0 3,389,063 ELETROHEMHAL MACHINING USING A(IHLUTRHDE ELECTRQLYTE llNCLUDING 2- Mllli.CAPZTQBENZGTHHAZOLE ORZ-BENZ- EVHBAZULETHEGL Mitchell A. La iioda, East Detroit, and Warren E.Duty,

Royal Oak, Mich assignors to General Motors Corporation, Detroit, Mich,a corporation or Delaware No Drawing. Filed (let. 23, 1965, Ser. No.504,147 12 Claims. (Cl. 204-143) This invention relates toelectrochemical machining processes and more particularly toelectrolytes for use therewith.

In recent years electrolytic machining procedures for generating shapes,cavities and contoured surfaces have been developed and are generallyclassified into one of two basic categories, the first beingelectrochemical machining and the second electrolytic grinding, aspecialized application of the first. Electrolytic grinding isessentially an electrochemical dcplating process which can be used onvirtually any electrically conductive material. It is generally adaptedto metal removel operations comparable to those performed by cutofiwheels, saws, and grinding or milling machines and the like and usesequipment similar in appearance to conventional grinders except for theelectrical accessories. About 95% of the metal removal results fromelectrolytic rather than mechanical action.

A particular version of an electrolytic grinding process ischaracterized by a flow of electrolyte between the workpiece and arotating grinding cathode wheel. The rotating cathode wheel consists ofa conductive metal matrix having a plurality of nonconducting abrasiveparticles imbedded therein to provide nonconductive spacing between theworkpiece and the cathodic matrix. Electric current is passed throughthe workpiece, electrolyte and cathode to dissolve the anodic surfacesof the workpiece, and the imbedded particles of the wheel abrade thesurface to remove any irregularities resulting from nonuniform erosionor reaction product buildup.

While aqueous solutions of individual inorganic salts, such as nitrates,cyanides, carbonates, hydroxides and nitrites have been used aselectrolytes in electrochemical machining and grinding processes, nonehas offered any significant advantage over the now well accepted aqueoussodium chloride solution most commonly used today. However, regardlessof What salt is chosen, an inherent problem with electrochemicalmachining and grinding processes using these salts singularly or incombination is overcut or wild-cutting which is the uncontrolled anodicdissolution of the workpiece in unwanted areas resulting in undesirabletapering of holes, rounding of edges, and the like. Such anodicdissolution can occur" even in areas which are fairly Well removed fromthe cathode. This wildcutting, or cutting in low current density areaswhich are bathed in the electrolyte, but substantially removed from thecathode, has been substantially reduced in the prior art by the use ofcostly and time consuming masking operations which isolate the areas tobe machined by protecting the surrounding areas from the erosive effectof the electrolyte. These masking operations are frequently quiteinvolved and require a high degree of skill to insure a satisfactoryproduct. Likewise, additional steps subsequent to the machining stepsare required to strip the workpiece of the mask. Additionally, the priorart has attempted to reduce wildcutting by designing special purposeelectrodes and machines to meet individual and specialized machiningrequirements.

By our invention we have at least reduced, and in most 3,389,068Patented June 18, 1968 cases actually eliminated, the need for recourseto the prior arts attempted resolutions.

It is therefore an object of our invention to provide a self-maskingelectrolyte.

It is a further object of our invention to provide an additive forexisting ECM electrolytes, which selectively inhibits anodic dissolutionin unwanted areas.

It is a further object of our invention to effect a sharply contouredmachining utilizing a basic aqueous electrolyte containing known saltsand improving same by adding thereto a compound which upon reaction withthe workpiece forms a film thereover, which film inhibits or stops offelectrolytic action or anodic dissolution in unwanted areas.

It is a further object of our invention to efiect a sharply contouredmachining by utilizing electrolytesv contain ing known salts and anadditive from the group consisting of electrochemical erosion inhibitingfilm forming thiazoles and thiols and particularlyZ-mercaptobenzothiazole (C H SCSl-LN) and 2-benzimidazolethiol (C HNC(SH) :N) respectively.

Further objects and advantages of the present invention will becomeapparent from the following detailed description of the invention.

Our invention, briefly stated, involves adding certain electrochemicalerosion inhibiting film forming thiazoles or thiols to basic known ECMelectrolytes. When added to these electrolytes the additives of ourinvention, and especially 2-mercaptobcnzothiazole or2-benzimidazolethiol, create an inhibited solution which effectivelyforms a heavy adherent film over the surface of the workpiece. The filmretards or substantially eliminates electrochemical erosion in thoseareas protected by the film. In a particular application of ourinvention a chloride electrolytic grinding electrolyte is modified byadding thereto either Z-mercaptobenzothiazole or Z-benzimidazolethiol.The film formed is subsequently abraded away in those areas whereelectrochemical machining is to continue, hence presenting a limiteduninhibited surface to unrestricted electrochemical action.

Our experience has been that using the additives of our invention, wecan successfully machine metal samples ranging from the softer lowercarbon steels (SAE 1008) to the harder low alloy steels (SAE 5160H). Theadditives of our invention are likewise applicable and effective forferrous alloys wherein the iron content is 50% or more.

While the preferred electrolytes comprise 80 grams per liter ofZ-mercaptobenzothiazole or 60 grams per liter of Z-benzimidazolethiol,respectively, with a 108 grams per liter aqueous sodium chloridesolution, we have found that effective electrolytes can be compoundedusing from 21 to 101 grams per liter of either Z-benzimidazolet-hiol orZ-mercaptobenzothiazole with dilute up to saturated solutions of sodiumchloride. In this connection the lighter alkali metal (Li, Na and K)chlorides are preferred because they produce relatively neutral pHs, donot plate out or have a deleterious elTect upon the cathode andrepresent a source of inexpensive material. Likewise, while electrolytesdilute as to chloride ion are operative, as a practical matter there areno significant advantages to operating at the lower concentrations and,in fact, it is less desirable to do so when considering such factors assolution conductivity and the like.

We have been successful in operating these electrolytes at voltages upto 40 volts and anode current densities from 5 to 500 amperes per squareinch. However, it is to be expected that in some circumstances evenhigher current densities can be used. Though room temperatures aregenerally preferred for manufacturing processes, we

assaees have successfully produced good results at higher temperatures.

Inasmuch as neither 2-benzimidazolethiol or Z-mercaptobenzothiazole arereadily soluble in water or sodium chloride solutions, it is necessaryfirst to prepare the respective compounds for introduction into therespective solutions. A mole-weight to mole-weight ratio ofZ-mercaptobenzothiazole or Z-benzimidazolethiol is mixed with sodiumhydroxide and subsequently dissolved in a limited amount of water whichis maintained at near its boiling point. We have successfully obtainedconcentrations up to 0.15 gram per milliliter using this procedure. Inorder to make up the preferred electrolyte the appropriate volume ofsaid 0.15 gram per milliliter solution is added to the appropriateaqueous sodium chloride solution. Owing to the ready availability ofacidic Z-mercaptobenzothiazole and Z-benzimidazolethiol theaforementioned procedure is preferred though it is recognized that thesodium salt of the respective compounds is formed and would be equallyeffective as an additive in that form.

Generally speaking, tests were conducted utilizing a system whereinanodic steel tube samples were brought up to a cathodic rotatingsintered bronze diamond impregnated wheel. A gravity feed system keptthe samples at the face of the wheel at all times. The feed system wassuch that an adjustable weight provided the capability of varying thepressures at which the samples would en gage the wheel. It was foundthat to properly evaluate the inhibitive efforts of our additives aminimum workpiece-to-wheel pressure should be employed in order toreduce the mechanical cutting component of the abrasive wheel. Theelectrolyte was pumped at a pressure of 9 pounds per square inch througha bore in the workpiece and into the gap between the cathode and theworkpiece at a rate of 0.25 gal. per minute. This gap was held constantat about 0.0025 inch by the spacer effect of the nonconductive diamondchips. Tube length decrease per unit time was used to determine metalremoval rates. The tube ends Were compared with those produced byelectrochemically grinding similar samples under the same conditions butwith additive-free electrolytes.

The following are some specific examples encompassed within the scope ofour invention:

Example 1 A room temperature electrolyte comprising 179 grams per literof sodium chloride, 33 grams per liter of 2-mercaptobenzothiazole andthe balance water, was used to machine a sample of an SAE 1020 steel. Avoltage of 14.5 volts was applied and a current density of 400 amperesper square inch was maintained resulting in the production of a sampleexhibiting a strong inhibiting film formed over the workpiece.

Example 2 A room temperature electrolyte comprising 197 grams per literof sodium chloride, 21 grams per liter of Z-mercaptobenzothiazole andthe balance water, was used to machine a sample of an SAE 1020 alloy. Avoltage of 8.9 volts was applied and a current density of 425 amperesper square inch was maintained, resulting in the production of a sampledisplaying a strong inhibiting film over the surface thereof.

Example 3 A room temperature electrolyte comprising 111 grams per literof sodium chloride, 77 grams per liter of Z-mercaptobenzothiazole andthe balance water, was used to machine a sample of an SAE 5160H alloy. Avoltage of 4.5 volts was applied and a current density of 165 ampercsper square inch was maintained, resulting in the production of a sampledisplaying a sharply cut machining, an incident of the production of ahighly inhibitive film over the surface of the workpiece.

(.1. Example 4 A room temperature electrolyte comprising 189 grams perliter of sodium chloride and 26.35 grams per liter ofZ-benzimidazolethiul and the balance water, was used to machine a sampleof an SAE 5l6OH alloy. A voltage of 9 volts was applied and a currentdensity of 360 amperes per square inch was maintained, resulting in theproduction of a sample displaying excellently machined characteristicswith no overcut.

Example 5 A room temperature electrolyte comprising 137 grams per literof sodium chloride, 60 grams per liter of 2- benzimidiazolethiol and thebalance water, was used to machine a sample of an SAE SlOH alloy. Avoltage of 4 volts was applied and a current density of 150 amperes persquare inch was maintained, resulting in the production of a sampledisplaying a strongly adherent inhibitive film formed over the surfacethereof.

Example 6 A room temperature electrolyte comprising 103 grams per literof sodium chloride, 83 grams per liter of 2- benzimidazolethiol and thebalance water, was used to machine a sample of an SAE 5160H alloy. Avoltage of 9 volts was applied and a current density of 400 amperes persquare inch was maintained, resulting in the production of a sampledisplaying excellent machined characteristics. I

While metal removal rates as high as 0.103 inch per minute were noted atrelatively high current densities, entirely satisfactory results wereconsistently produced at lower current densities. Likewise, thoughelectrolytes were compounded using as much as 101 grams per liter ofeither Z-mercaptobenzothiazole or 2-benzimidazolethiol, no appreciableadvantages over the somewhat lesser concentrated solutions were readilyapparent save for the somewhat faster formation of a stronger film. Itis doubt ful that the marginal extra benefits to be derived from thehighly concentrated solutions would ever economically justify their useover the preferred ran es.

Other nonabrasive means for the local removal of the inhibiting film ofour invention may be employed, such as washing away with localizedincreased electrolyte flow, and/or variety of sophisticated variationsof these and others. Therefore, though our invention has been describedin terms of certain preferred embodiments, it is to be understood thatothers may be adopted and that the scope of our invention is not limitedexcept by the appended claims.

We claim:

1. An aqueous electrochemical machining electrolyte consistingessentially of an alkali metal chloride and from about 21 grams perliter to about 101 grams per liter of an additive from the groupconsisting of Z-benzimidazolethiol and Z-mercaptobenzothiazole.

2. An electrolyte in accordance with claim 1 wherein said alkali metalis from the group consisting of lithium, sodium and potassium.

3. An electrolyte in accordance with claim 1 wherein said additive isZ-oenzimidazolethioi.

4. An electrolyte in accordance with claim 3 wherein the concentrationof the Z-oenzimidaz lethiol is from about 21 grams per liter to about 60grams per liter.

5. An electrolyte in accordance with claim 1 wherein said additive isZ-mercaptobenzothiazole.

6. An electrolyte in accordance with claim 5 wherein the concentrationof said Z-mercaptobenzothiazole is from about 21 grams per liter toabout 80 grams per liter.

7. A process for electrochemically machining ferrous metals comprisingthe steps of establishing said metal as the anode in an electrochemicalcell oricrting an clectrolytic grinding cathode adjacent tosaid metal,bathing the junction between said cathode and said metal in an aqueouselectrolyte consisting essentially of an alkali metal asaaaaa chlorideand an additive from the group consisting of 2-benzimidazolethiol andZ-mercaptobenzothiazole, passing an electric current through said metal,electrolyte and cathode whereby an electrochemical erosion inhibitingfilm is formed on the metal, and selectively removing said film wherebyelectrochemical machining can thus selectively continue.

8. A process in accordance with claim 7 wherein the concentration ofsaid additive is from about 21 grams per liter to about 101 grams perliter.

9. A process in accordance with claim 7 wherein said additive is2-benzimidazolethiol.

10. A process in accordance with claim 9 wherein the concentration ofsaid Z-benzimidazolethiol is from about 21 grams per liter to about 60grams per liter.

References Cited UNITED STATES PATENTS 2,939,825 6/1960 Faust et a1.204-442 3,058,895 10/1962 Williams 204-143 3,130,138 4/1964 Faust et al.204143 3,284,327 11/1966 Maeda et al 204-143 r ROBERT K. MIHALEK,Primary Examiner. O

7. A PROCESS FOR ELECTROCHEMICALLY MACHINE FERROUS METALS COMPRISING THESTEPS OF ESTABLISHING SAID METAL AS THE ANODE IN AN ELECTROCHEMICAL CELLORIENTING AN ELECTROLYTIC GRINDING CATHODE ADJACENT TO SAID METAL,BATHING THE JUNCTION BETWEEN SAID CATHODE AND SAID METAL IN AN AQUEOUSELECTROLYTE CONSISTING ESSENTIALLY OF AN ALKALI METAL CHLORIDE AND ANADDITIVE FROM THE GROUP CONSISTING OF 2-BENZIMIDAZOLETHIOL AND2-MERCAPTOBENZOTHIAZOLE, PASSING AN ELECTRIC CURRENT THROUGH SAME METAL,ELECTROLYTE AND CATHODE WHEREBY AN ELECTROCHEMICAL EROSION INHIBITINGFILM IS FORMED ON THE METAL, AND SELECTIVELY REMOVING SAID FILM WHEREBYELECTROCHEMICAL MACHINING CAN THUS SELECTIVELY CONTINUE.